What happened
On the morning of 7 October 2014, a Robinson R44 II helicopter, registration ZK-HBQ, crashed into dense bush near Mt Arthur in New Zealand's Kahurangi National Park. The aircraft was being ferried from Karamea to Nelson for scheduled maintenance. Despite forecast high winds and turbulence, the pilot proceeded with the flight after receiving reports from another aircraft that wind speeds in the area were manageable.
During the flight, the helicopter crossed the Tasman Ranges. While navigating the area, the aircraft experienced a catastrophic failure when one of the main rotor blades struck the cabin. This impact caused the main rotor assembly to separate from the fuselage, leading to the total disintegration of the aircraft in mid-air. The pilot, the sole occupant, was killed in the accident.
The investigation
Investigators from the Transport Accident Investigation Commission (TAIC) examined the wreckage and analyzed flight tracking data from the TracPlus system and an on-board GPS unit. The investigation included a metallurgical analysis of a rotor blade section and interviews with the pilot's flight instructor and examiner.
Technical experts from the Robinson Helicopter Company also assisted in reviewing the wreckage. The investigation focused on determining whether the structural failure was caused by low rotor RPM or a phenomenon known as 'mast-bumping.' Data recovered from the GPS unit provided the precise flight path and altitude leading up to the impact.
Findings
- The primary cause of the in-flight break-up was mast-bumping, a phenomenon where the rotor blades contact the mast due to excessive teetering movement.
- The accident was triggered when the helicopter entered a low-G condition, likely caused by severe downdraughts and turbulence while crossing the Tasman Ranges.
- The pilot was flying a relatively new type rating, having only 11 hours of experience on the R44 model.
- At the time of the accident, the pilot was flying through an area with forecast high winds, which could have contributed to the low-G state.
- The investigation noted that Robinson helicopters are uniquely susceptible to rapid right-hand rolls during severe low-G events, which can lead to structural failure before a pilot can react.
Safety action
- The Commission recommended that the FAA extend wind and turbulence limitations, currently applied to the R22, to the R44 and R66 models.
- It advised the Director of Civil Aviation to implement these same limitations for all Robinson helicopter pilots in New Zealand, regardless of experience level.
- The inquiry highlighted the necessity for pilots to use the approved 'aft cyclic' recovery technique immediately upon sensing low-G conditions to prevent mast-bumping.